Known methods for obtaining a single crystal silicon ingot include a floating zone melting process (FZ process) and a Czochralski process (CZ process). The CZ process is used commonly. When manufacturing a silicon single crystal or the like by the CZ process, a crucible apparatus is used. The crucible apparatus comprises an outer crucible that is heated by a heater or the like, and an inner crucible in which raw materials such as a silicon single crystal are accommodated. Usually, a crucible made of quartz is employed for the inner crucible from the viewpoint of the reactivity with silicon and the purity, whereas a crucible made of graphite is employed for the outer crucible from the viewpoint of the purity, the heat resistance, and the strength. The crucible apparatus is used for manufacturing silicon single crystals and the like in the state in which the inner crucible is inserted in the outer crucible.
The above-described crucible apparatus generally has the problems as shown in the following (1) and (2).    (1) The inner crucible made of quartz is heavy in weight and low in impact resistance. Therefore, extra care must be taken when inserting the inner crucible into the outer crucible. This leads to the problem of poor workability. Another problem is that, since there is a difference in thermal expansion coefficient between the inner crucible and the outer crucible, damages such as cracks are caused in both crucibles when cooling the crucibles after finishing the manufacture of a silicon single crystal or the like. Still another problem is that SiO gas or the like generated from the quartz crucible causes a reaction with the outer crucible and silicon-carbidization and thickness decrease occur in the outer crucible.
To resolve such problems, it has conventionally been proposed that a member for protecting the outer crucible and the inner crucible from damages should be provided between the crucibles ([a] and [b] below).    [a] One proposal is as follows. A sheet or fabric in which pyrocarbon is coated on the surface of a carbon fiber member, such as textile made of carbon fiber, has been disclosed. A proposal is that the sheet or fabric should be disposed between the outer crucible and the inner crucible. (See, for example, Patent References 1 and 2 listed below.)
It has been described that according to such a configuration, the sheet or the like can function as a cushioning material since the sheet or the like has a certain degree of flexibility, and that the reaction of the SiO gas or the like generated from the inner crucible with the outer crucible can be prevented since the pyrocarbon coated on the surface reacts with the SiO gas or the like.    [b] Another proposal is as follows. An expanded graphite sheet, in which expanded graphite is formed into a sheet shape, is superior in flexibility and compressibility to the carbon fiber member. Such an expanded graphite sheet should be used as a crucible protection sheet. (See, for example, Patent References 3 to 5 listed below.)
The expanded graphite sheet is a material that has flexibility and shows high compression ratio and high recovery ratio. It also shows good planar thermal conduction capability. Therefore, it is effective to uniformize the temperatures along vertical directions of the crucibles, and it also serves to alleviate heat impact and expansion-contraction stress of thermal expansion. Moreover, Patent Reference 3 describes that the expanded graphite sheet is resistant to gas permeability because it is a highly anisotropic material with very low gas permeability.    (2) In order to improve the quality of the single crystal silicon manufactured by the CZ process, it is required that the purity of polycrystalline silicon, which is the raw material, be as high as possible, or in other words, the amount of impurities be as small as possible.
Nevertheless, in the actual manufacturing of single crystal silicon, a trace amount of impurity as a dopant element is added to the raw material, the polycrystalline silicon, in order to obtain a single crystal silicon with desired electric characteristics, and the single crystal silicon is manufactured from the polycrystalline silicon containing such a dopant element.
Here, if the amount of the dopant element existing in the molten silicon is not controlled, an abnormality will occur in the quality of the manufactured single crystal silicon. During the manufacture, impurities may evaporate from the crucible apparatus used for melting the polycrystalline silicon, specifically, from the inner crucible, the outer crucible, and the sheet disposed between the inner and outer crucibles. It is feared that these impurities may contaminate the molten silicon.
It is impossible to control the amount of such impurities originating from the crucible apparatus. Therefore, currently, it has been proposed that the possibility of contamination of the molten silicon by impurities should be lowered by minimizing the amount of impurities contained in the crucible apparatus itself (see Patent References 6 to 8 listed below).
[Patent Reference 1] Japanese Published Unexamined Patent Application No. 2001-261481
[Patent Reference 2] Japanese Published Unexamined Patent Application No. 2002-226292
[Patent Reference 3] Japanese Patent No. 2528285
[Patent Reference 4] Japanese Published Unexamined Patent Application No. 2003-267781
[Patent Reference 5] Japanese Published Unexamined Patent Application No. 2004-75521
[Patent Reference 6] Japanese Published Examined Patent Application No. H6-2637
[Patent Reference 7] Japanese Patent No. 2923260
[Patent Reference 8] Japanese Patent No. 3410380